This application claims the priority of Japanese Patent Applications No. 227243/1993 filed on Sep. 13, 1993, No. 279141/1993 filed on Nov. 9, 1993 and No. 118466/1994 filed on May 31, 1994, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a power inverter, and more particularly to a power inverter for use in interconnecting a direct-current supply and a commercial alternating-current supply so that the power may be supplied by both of them to a load.
2. Description of the Prior Art
System interconnection of the kind indicated above is recommended from the standpoint of economy of energy, such system interconnection being particularly suitable for employment in those cases where a direct-current supply such as a solar battery or a fuel cell is connected to a commercial alternating-current supply through a power inverter so that electrical energy may be supplied by both of them to a load such as a household electric appliance.
In order to insure the safe prosecution of maintenance work to be done for a commercial alternating-current supply system involved in a system interconnection, the connection between the power inverter and the commercial alternating-current supply must be cut when an accidental power failure happens or a scheduled power cut takes place in the commercial alternating-current supply system. Therefore, it is necessary to provide a means for cutting the connection therebetween at the time of the above-mentioned power failure or power cut so as to prevent the direct-current supply system from becoming an independent current supply.
When a transition occurs from the state in which two current supplies are concurrently operative to the state in which only one current supply is operative, a marked variation in the voltage level or frequency at a junction point between the power inverter and the commercial alternating-current supply is caused thereby with a fair degree of certainty. The connection therebetween may be cut when this variation is detected. However, in a state of equilibrium in which the output power of the power inverter is nearly equal to the demand of the load, the voltage level or frequency at the above-mentioned junction point hardly undergoes a variation and, consequently, a power failure or power cut in the commercial alternating-current supply system cannot be detected.
In a known method of detecting the power failure or power cut, a band-pass filter for separating a fundamental frequency component from the output voltage of an inverter circuit is installed in a feedback path provided for controlling the output current of the inverter circuit. The band-pass filter has a center frequency which deviates from the commercial frequency by several Hz so that the frequency of an output to be produced by the power inverter at the time of a power failure or power cut may be allowed to vary from the commercial frequency to the center frequency of the band-pass filter.
Under the normal condition, the frequency of the voltage at the above-mentioned junction point is held at the commercial frequency by the commercial alternating-current supply. The band-pass filter having a center frequency deviating from the commercial frequency causes a phase difference between the output current and the output voltage of the power inverter. This phase difference in turn causes a decrease in the power factor of the power inverter. The power failure or power cut cannot be detected when the state of equilibrium is caused by the absolute value of the power factor of the load which is equal to the absolute value of the power factor of the power inverter.